It is well known in the art of semiconductor manufacturing that depth of focus (“DOF”) is an important issue. Fukuda (Hitachi Central Research Labs) proposed a method to increase DOF using FLEX (Focus Latitude Enhanced eXposure) in 1989, wherein the exposure is performed using two stage focal positions. This is discussed in U.S. Pat. No. 4,869,999, entitled METHOD OF FORMING PATTERN AND PROJECTION ALIGNER FOR CARRYING OUT THE SAME, issued to Fukuda, et al on Sep. 26, 1989 (“Fukuda I”), where the specification also notes that:                It has been found by the inventors' investigation that the effective focal depth of an exposure optical system can be increased by overlapping a plurality of light beams having image points at different positions on an optical axis, and thus the image of a mask pattern can be formed accurately in a region between the top and the bottom of the topography of a substrate surface. The term “image point” indicates a point on the conjugate plane of the mask pattern with respect to the exposure optical system. Accordingly, when an exposure operation for exposing a substrate coated with a resist layer to exposure light through a mask is performed a plurality of times at different positional relations in the direction of the optical axis between the resist layer and the image plane of a mask pattern, or when exposure operations at the different positional relations are simultaneously performed, the image of the mask pattern can be accurately formed not only at the top and the bottom of the topography of a substrate surface but also at an intermediate position between the top and the bottom of the topography. Thus, a fine pattern can be formed accurately all over the topography. (Col. 3, lines 33–54, emphasis added) Fukuda I also states:        Furthermore, in the present embodiment, the image plane of a mask pattern was formed at two different positions in (or over) the substrate by displacing the substrate in the direction of an optical axis. Alternatively, the image plane of the mask pattern may be formed at different positions by moving a reticle having a mask pattern in the direction of the optical axis, by introducing a transparent material different in refractive index from air into an exposure optical system, by changing the atmospheric pressure in the whole or a portion of the exposure optical system, by using a lens having a multiple focal point, by overlapping light beams from a plurality of exposure optical systems which form the image plane of a mask pattern in different planes, or by using different wavelengths or a continuous wavelength in the same exposure optical system. (Col. 6, lines 37–53, emphasis added) It has also been proposed, e.g., in systems sold, e.g., by Nikon, that a stepper allow continuous stage motion between two focal planes.        
In U.S. Pat. No. 4,937,619, entitled PROJECTION ALIGNER AND EXPOSURE METHOD, issued to Fukuda, et al. on Jun. 26, 1990 (“Fukuda II”), there is proposed a system in which separate laser beams are generated and optically combined to produce a single beam with a plurality of different wavelengths arriving at the reticle in the lithography tool at the same time. Fukuda II also notes:                FIG. 5 is a configuration diagram of a third embodiment of the present invention. The embodiment shown in FIG. 5 comprises a reflecting mirror 31, an etalon 32, an excimer laser gas cavity 33, an output mirror 34, a mirror 35, an etalon angle control circuit 36, a laser oscillation control circuit 37, an exposure wavelength control circuit 38, an illumination optical system 14, a reticle 15, a projection lens 16, a substrate stage 17, and various elements required for the projection aligner.        The etalon 32 narrows the bandwidth of the laser beam oscillated by the excimer laser resonator composed of a reflecting mirror 31, excimer laser gas cavity 33, and an output mirror 34, and changes the central wavelength of light narrowed in bandwidth by adjusting the angle of the etalon 32 minutely. The wavelength control circuit 38 sends a command to the etalon angle control circuit 36 to set the angle of the etalon at a predetermined value, and sends a command to the laser oscillation control circuit 37 to cause laser oscillation with a predetermined number of exposure pulses for the etalon angle. The exposure wavelength control circuit 38 is capable of changing the set angle of the etalon 32 during the exposure of one exposure region located on the substrate by using the above described function and is capable of performing projection exposure by using light having a plurality of different wavelengths. Since the projection lens 16 focuses the pattern on the reticle 15 onto a different position on an identical optical axis with respect to each of the above-described plurality of wavelengths, it is possible to perform the focus latitude enhancement exposure by using the present projection aligner.        Instead of being disposed between the reflecting mirror 31 and the laser resonator 33 as shown in FIG. 5, the etalon 32 and the wavelength control means may be disposed between the output mirror 34 and the laser gas cavity 33, or between the output mirror 34 and the illumination optical system 14, for example. Further, the above described line narrowing and wavelength alteration are not restricted to the method of changing the angle of the etalon.        The present embodiment is economically advantageous because only one excimer laser is used. In addition, lowering of laser output caused by bandwidth narrowing can be limited to a small value because the bandwidth-narrowing device is disposed between the reflecting mirror and the output mirror.        By using the present projection aligner, it was confirmed that the depth of focus of fine patterns increased in the same way as the first embodiment.        
In U.S. Pat. No. 5,303,002, entitled METHOD AND APPARATUS FOR ENHANCING THE FOCUS LATITUDE IN LITHOGRAPHY, issued to Yan on Apr. 12, 1994, there is proposed also combining separately generated laser beams to obtain a single beam at the reticle with a plurality of wavelengths. Yan also proposes the generation of three output beams from a single laser system, but the embodiment proposed is not workable.
In the prior applications assigned to applicant's assignee referenced above “spectral engineering” has been proposed using, e.g., a wavelength and bandwidth tuning mechanism to produce an apparent spectrum over a series of pulses in a burst of pulses output by the laser system that effectively contains a plurality of discrete spectra. The '280 patent and '773 application suggest that:                A fast responding tuning mechanism is then used to adjust center wavelength of laser pulses in a burst of pulses to achieve an integrated spectrum for the burst of pulses approximating the desired laser spectrum. The laser beam bandwidth is controlled to produce an effective beam spectrum having at least two spectral peaks in order to produce improved pattern resolution in photo resist film. . . . In a preferred embodiment, a wavelength tuning mirror is dithered at dither rates of more than 500 dithers per second in phase with the repetition rate of the laser. . . . In another embodiment, the maximum displacement was matched on a one-to-one basis with the laser pulses in order to produce a desired average spectrum with two peaks for a series of laser pulses. Other preferred embodiments utilize three separate wavelengths.        
The disclosures of the foregoing issued patents are hereby incorporated by reference.
RELAX, an acronym for “Resolution Enhancement by Laser-spectrum Adjusted eXposure”, according to one embodiment, is based on the concept of engineering the laser spectrum to have two (or more) peaks in order to increase depth of focus (DOF). Simulations using PROLITH software have demonstrated that a dual peak spectral shape can improve DOF two- to three-fold with acceptable sacrifice to exposure latitude, e.g., for specific configurations of contact holes and line spacing patterns. The light spectrum can contain, e.g., two spectral peaks (either simultaneously or in alternating pulses) which create two different optimal focal planes in the photoresist. This puts every part of even very thick resists (400 nm and up) within acceptable depth of focus at a given reasonable (6–12%) exposure latitude. As a result of exposure with dual peaks, the resist sidewalls maintain an acceptable angle through the resist and there is good control on critical dimensions. Fewer clogged up or underexposed contact holes deliver improved yield and increased profitability for the chipmaker. According to aspects of an embodiment of the present invention applicants have provided for a solution that provides lot-to-lot control on the peak-to-peak separation. Optimal separation can be determined through simulation using CD, illumination condition, mask information and laser spectrum. Applicants proposal also provides for the sum of the integration of the energies under the split spectra will be equal to the total integration of the energy from an initial (non-split) spectrum and peaks that can be made symmetrically separated from the central wavelength.
Specific implementations of RELAX, require modifications to existing laser light source systems, e.g., gas discharge laser light source systems, e.g., in actual implementation of wavefront splitting, metrology, synchronization with the application using the light, e.g., a wafer scanner scanning window, and like issues addressed by aspects of embodiments of the present invention.